likely the noise is getting in from the cars electric system, some times wires are to close to computer control units n such. you could check if wires are routed close to obvious components.
if the speakers are factory new speakers might help.
you can also add a small capacitor inline on the positive speaker wires i am not sure of the value right now but it is a cap for a 6db passive crossover.
the radio could all so be the culprit.